[Tarantool-patches] [RFC] Quorum-based synchronous replication
Aleksandr Lyapunov
alyapunov at tarantool.org
Tue Apr 7 16:02:25 MSK 2020
On 4/4/20 12:08 AM, Sergey Ostanevich wrote:
> * **Status**: In progress
> * **Start date**: 31-03-2020
> * **Authors**: Sergey Ostanevich @sergos \<sergos at tarantool.org\>
> * **Issues**:
>
> ## Summary
>
> The aim of this RFC is to address the following list of problems
> formulated at MRG planning meeting:
> - protocol backward compatibility to enable cluster upgrade w/o
> downtime
> - consistency of data on replica and leader
> - switch from leader to replica without data loss
> - up to date replicas to run read-only requests
> - ability to switch async replicas into sync ones
> - guarantee of rollback on leader and sync replicas
> - simplicity of cluster orchestration
>
> What this RFC is not:
>
> - high availability (HA) solution with automated failover, roles
> assignments an so on
> - master-master configuration support
>
>
> ## Background and motivation
>
> There are number of known implemenatation of consistent data presence in
> a cluster. They can be commonly named as "wait for LSN" technique. The
> biggest issue with this technique is the abscence of rollback gauarantees
> at replica in case of transaction failure on one master or some of the
> replics in the cluster.
>
> To provide such capabilities a new functionality should be introduced in
> Tarantool core, with limitation mentioned before - backward compatilibity
> and ease of cluster orchestration.
>
> ## Detailed design
>
> ### Quorum commit
> The main idea behind the proposal is to reuse existent machinery as much
> as possible. It will ensure the well-tested and proven functionality
> across many instances in MRG and beyond is used. The transaction rollback
> mechanism is in place and works for WAL write failure. If we substitute
> the WAL success with a new situation which is named 'quorum' later in
> this document then no changes to the machinery is needed. The same is
> true for snapshot machinery that allows to create a copy of the database
> in memory for the whole period of snapshot file write. Adding quorum here
> also minimizes changes.
>
> Currently replication represented by the following scheme:
> ```
> Customer Leader WAL(L) Replica WAL(R)
> |------TXN----->| | | |
> | | | | |
> | [TXN Rollback | | |
> | created] | | |
> | | | | |
> | |-----TXN----->| | |
> | | | | |
> | |<---WAL Ok----| | |
> | | | | |
> | [TXN Rollback | | |
> | destroyed] | | |
> | | | | |
> |<----TXN Ok----| | | |
> | |-------Replicate TXN------->| |
> | | | | |
> | | | [TXN Rollback |
> | | | created] |
> | | | | |
> | | | |-----TXN----->|
> | | | | |
> | | | |<---WAL Ok----|
> | | | | |
> | | | [TXN Rollback |
> | | | destroyed] |
> | | | | |
> ```
>
>
> To introduce the 'quorum' we have to receive confirmation from replicas
> to make a decision on whether the quorum is actually present. Leader
> collects necessary amount of replicas confirmation plus its own WAL
> success. This state is named 'quorum' and gives leader the right to
> complete the customers' request. So the picture will change to:
> ```
> Customer Leader WAL(L) Replica WAL(R)
> |------TXN----->| | | |
> | | | | |
> | [TXN Rollback | | |
> | created] | | |
> | | | | |
> | |-----TXN----->| | |
> | | | | |
> | |-------Replicate TXN------->| |
> | | | | |
> | | | [TXN Rollback |
> | |<---WAL Ok----| created] |
> | | | | |
> | [Waiting | |-----TXN----->|
> | of a quorum] | | |
> | | | |<---WAL Ok----|
> | | | | |
> | |<------Replication Ok-------| |
> | | | | |
> | [Quorum | | |
> | achieved] | | |
> | | | | |
> | [TXN Rollback | | |
> | destroyed] | | |
> | | | | |
> | |----Quorum--->| | |
> | | | | |
> | |-----------Quorum---------->| |
> | | | | |
> |<---TXN Ok-----| | [TXN Rollback |
> | | | destroyed] |
> | | | | |
> | | | |----Quorum--->|
> | | | | |
> ```
>
> The quorum should be collected as a table for a list of transactions
> waiting for quorum. The latest transaction that collects the quorum is
> considered as complete, as well as all transactions prior to it, since
> all transactions should be applied in order. Leader writes a 'quorum'
> message to the WAL and it is delivered to Replicas.
I think we should cal the the message something like 'confirm'
(not 'quorum'), and mention here that it has its own LSN.
Besides, it's very similar to phase two of two-phase-commit,
we'll need it later.
>
> Replica should report a positive or a negative result of the TXN to the
> Leader via the IPROTO explicitly to allow Leader to collect the quorum
> or anti-quorum for the TXN. In case negative result for the TXN received
> from minor number of Replicas, then Leader has to send an error message
> to each Replica, which in turn has to disconnect from the replication
> the same way as it is done now in case of conflict.
I'm sure that unconfirmed transactions must not be visible both
on master and on replica since the could be aborted.
We need read-committed.
>
> In case Leader receives enough error messages to do not achieve the
> quorum it should write the 'rollback' message in the WAL. After that
> Leader and Replicas will perform the rollback for all TXN that didn't
> receive quorum.
>
> ### Recovery and failover.
>
> Tarantool instance during reading WAL should postpone the commit until
> the quorum is read. In case the WAL eof is achieved, the instance should
> keep rollback for all transactions that are waiting for a quorum entry
> until the role of the instance is set. In case this instance become a
> Replica there are no additional actions needed, sine all info about
> quorum/rollback will arrive via replication. In case this instance is
> assigned a Leader role, it should write 'rollback' in its WAL and
> perform rollback for all transactions waiting for a quorum.
>
> In case of a Leader failure a Replica with the biggest LSN with former
> leader's ID is elected as a new leader. The replica should record
> 'rollback' in its WAL which effectively means that all transactions
> without quorum should be rolled back. This rollback will be delivered to
> all replicas and they will perform rollbacks of all transactions waiting
> for quorum.
>
> ### Snapshot generation.
>
> We also can reuse current machinery of snapshot generation. Upon
> receiving a request to create a snapshot an instance should request a
> readview for the current commit operation. Although start of the
> snapshot generation should be postponed until this commit operation
> receives its quorum. In case operation is rolled back, the snapshot
> generation should be aborted and restarted using current transaction
> after rollback is complete.
There is no guarantee that the replica will ever receive 'confirm'
('quorum') message, for example when the master is dead forever.
That means that in some cases we are unable to make a snapshot..
But if we make unconfirmed transactions invisible, the current
read view will give us exactly what we need, but I have no idea
how to handle WAL rotation ('restart') in this case.
>
> After snapshot is created the WAL should start from the first operation
> that follows the commit operation snapshot is generated for. That means
> WAL will contain a quorum message that refers to a transaction that is
> not present in the WAL. Apparently, we have to allow this for the case
> quorum refers to a transaction with LSN less than the first entry in the
> WAL and only once.
Not 'only once', there could be several unconfirmed transactions
and thus several 'confirm' messages.
>
> ### Asynchronous replication.
>
> Along with synchronous Replicas the cluster can contain asynchronous
> Replicas. That means async Replica doesn't reply to the Leader with
> errors since they're not contributing into quorum. Still, async
> Replicas have to follow the new WAL operation, such as keep rollback
> info until 'quorum' message is received. This is essential for the case
> of 'rollback' message appearance in the WAL. This message assumes
> Replica is able to perform all necessary rollback by itself. Cluster
> information should contain explicit notification of each Replica
> operation mode.
>
> ### Synchronous replication enabling.
>
> Synchronous operation can be required for a set of spaces in the data
> scheme. That means only transactions that contain data modification for
> these spaces should require quorum. Such transactions named synchronous.
> As soon as last operation of synchronous transaction appeared in Leader's
> WAL, it will cause all following transactions - matter if they are
> synchronous or not - wait for the quorum. In case quorum is not achieved
> the 'rollback' operation will cause rollback of all transactions after
> the synchronous one. It will ensure the consistent state of the data both
> on Leader and Replicas. In case user doesn't require synchronous operation
> for any space then no changes to the WAL generation and replication will
> appear.
>
> Cluster description should contain explicit attribute for each Replica
> to denote it participates in synchronous activities. Also the description
> should contain criterion on how many Replicas responses are needed to
> achieve the quorum.
>
>
> ## Rationale and alternatives
>
> There is an implementation of synchronous replication as part of gh-980
> activities, still it is not in a state to get into the product. More
> than that it intentionally breaks backward compatibility which is a
> prerequisite for this proposal.
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